Process for operating a cooling tower comprising the treatment of feed water by direct osmosis

ABSTRACT

A process for operating a cooling tower ( 10 ), said process comprising: introducing a feed solution into a cooling tower, evaporating solvent from the feed solution to produce a concentrated solution, removing a portion of the concentrated solution as blow-down, passing at least a portion of the blow-down through a first membrane in a nanofiltration unit ( 14 ), such that at least some of the solute in the blow-down are retained as a concentrated blow-down solution on the retentate-side of the first membrane, contacting at least a portion of the concentrated blow-down solution ( 42 ) with one side of a second membrane ( 12 ) and contacting a solution having a lower solute concentration than the combined concentrated blow-down solution and the return from the cooling tower with the opposite side of the second membrane, such that solvent flows across the second membrane to dilute the concentrated blow down solution by direct osmosis, and re-introducing the diluted blow-down solution to the cooling tower.

The present invention relates to a method of operating a cooling tower.

Heat exchangers are often used to remove excess heat from industrialprocesses. Typical heat exchangers include shell and tube-type heatexchangers, which comprise a length of tubing partially enclosed withina housing or shell. An industrial process stream containing excess heatis introduced into the tubing, whilst a coolant, such as water, ispassed through the shell via a separate inlet and outlet. The waterremoves excess heat from the process stream. Thus, the water exiting theshell is at a higher temperature than the coolant entering the shell.The heated water stream is cooled in a cooling tower before it isreintroduced into the shell. In this way, heat removal can be carriedout in a continuous manner.

Most cooling towers contain a porous filler material, known as decking.Water is introduced into the top of the cooling tower and drips downthrough the decking, whilst air is blown through the decking, causingsome of the water to evaporate. The loss of heat by evaporation(evaporative cooling) lowers the remaining water temperature. The cooledwater is recirculated to the heat exchanger.

As evaporation occurs, contaminants, such as dissolved solids, build upin the recirculating water. Such contaminants can cause fouling, forexample, as a result of biological growth, scale formation, corrosionand/or sludge deposition. The contaminant level may be reduced byremoving a portion of the recirculating water from the system. Theremoval of water in this manner is known as blow-down.

To replace the water loss from the system, make-up water is introducedinto the cooling tower. Various attempts have been made to treat thismake-up water to reduce the risk of fouling. In WO 2005/120688, forexample, the make-up water is formed by positioning a selectivelypermeable membrane between seawater and a clean solution of sodiumchloride having a higher solute concentration than the seawater. Thedifference in osmotic potential causes water from the seawater side ofthe membrane to flow across the membrane to dilute the sodium chloridesolution. This diluted sodium chloride solution is substantially freefrom the biological contaminants typically found in seawater and can betreated with anti-corrosion, anti-scaling and anti-fouling agents beforebeing used as the make-up water for the system.

According to the present invention, there is provided a process foroperating a cooling tower, said process comprising:

introducing a feed solution into a cooling tower,

evaporating solvent from the feed solution to produce a concentratedsolution,

removing a portion of the concentrated solution as blow-down,

passing at least a portion of the blow-down through a first membrane,such that at least some of the solute in the blow-down is retained as aconcentrated blow-down solution on the retentate-side of the firstmembrane,

contacting at least a portion of the concentrated blow-down solutionwith one side of a second membrane and contacting a solution having alower solute concentration than the concentrated blow-down solution withthe opposite side of the second membrane, such that solvent flows acrossthe second membrane to dilute the concentrated blow down solution bydirect osmosis, and

re-introducing the diluted blow-down solution to the cooling tower.

In a preferred embodiment, the process further comprises

positioning a membrane between a source solution and a draw solutionhaving a higher solute concentration than the source solution, such thatsolvent from the source solution flows across the membrane to dilute thedraw solution,

introducing the diluted source solution into the cooling tower as thefeed solution,

evaporating solvent from the feed solution to produce a concentratedfeed solution,

and, optionally, reusing at least a portion of the concentrated feedsolution as the draw solution.

The source solution may be an aqueous stream. Preferably, the aqueousstream is an impure stream, such as seawater, river water, lake water,rain water, brackish water and water from industrial process streams.Suitable industrial process streams may be derived from, for example,the salty residues of desalination plants, such as thermal desalinationand/or reverse osmosis plants. Aqueous effluents, such as thosetypically employed as make-up water for conventional cooling towers, mayalso be used.

Any suitable solution having a higher solute concentration than thesource solution may be used as the draw solution. The draw solution maybe have a known composition. For example, in one embodiment, the drawsolution is formed by introducing a known quantity of at least onesolute into a known quantity of solvent. Thus, the draw solution mayconsist essentially of a selected solute(s) dissolved in a selectedsolvent, such as water. By forming the draw solution in this manner, asubstantially clean solution may be produced. Thus, the draw solutionmay have a reduced concentration of suspended particles, biologicalmatter and/or other components that may cause fouling of the coolingsystem. In a preferred embodiment, the draw solution is substantiallyfree of suspended particles, biological matter and/or other componentsthat may cause fouling of the cooling system.

In one embodiment, additives, such as scale inhibitors, corrosioninhibitors, biocides and/or dispersants, are included in the drawsolution. By recirculating a portion of the draw solution through thecooling tower, these additives may be reused. Preferably, the bulk ofthe draw solution is recirculated in a closed loop, such that a largeproportion of the components of the draw solution are retained withinthe loop. Thus, once the draw solution is formed, it may not benecessary to continuously add fresh solute and/or additives to thesolution.

The solute (or osmotic agent) in the draw solution is preferably awater-soluble salt. Suitable salts include salts of ammonium and metals,such as alkali metals (e.g. Li, Na, K) and alkali earth metals (e.g. Mgand Ca). The salts may be fluorides, chlorides, bromides, iodides,sulphates, sulphites, sulphides, carbonates, hydrogencarbonates,nitrates, nitrites, nitrides, phosphates, aluminates, borates, bromates,carbides, chlorides, perchlorates, hypochlorates, chromates,fluorosilicates, fluorosilicates, fluorosulphates, silicates, cyanidesand cyanates. One or more salts may be employed. In a preferredembodiment, the solute of the second solution is a potassium, magnesiumor sodium salt in water.

As mentioned above, the source solution is placed on one side of amembrane, while the draw solution is placed on the opposite side of themembrane. As a result of the difference in osmotic potential between thesolutions, solvent passes across the membrane to dilute the drawsolution by direct osmosis. The flow occurs along a concentrationgradient. Thus, high pressures are not required to induce solvent flow.However, a pressure differential across the membrane may be applied, forexample, to increase the flux of solvent.

Any suitably selective membrane may be used in the direct osmosis step.An array of membranes may be employed. Suitable membranes includecellulose acetate (CA) and cellulose triacetate (CTA) (such as thosedescribed in McCutcheon et al., Desalination 174 (2005) 1-11) andpolyamide (PA) membranes. The membrane may be planar or take the form ofa tube or hollow fibre. Thin membranes may be employed, particularly,when a high pressure is not applied to induce solvent flow from thefirst solution to the second solution. If desired, the membrane may besupported on a supporting structure, such as a mesh support.

In one embodiment, one or more tubular membranes may be disposed withina housing or shell. The source solution may be introduced into thehousing, whilst the draw solution may be introduced into the tubes. Asthe solvent concentration of the source solution is higher than that ofthe draw solution, solvent will diffuse across the membrane from thesource solution into the draw solution. The diluted draw solution may berecovered from the interior of the tubes, whilst the concentrated firstsolution may be removed from the housing (or vice-versa).

When a planar membrane is employed, the sheet may be rolled such that itdefines a spiral in cross-section.

The pore size of the membrane may be selected depending on the size ofthe solvent molecules that require separation. It may be possible to usea membrane having a pore size that allows two or more different types ofsolvent molecules to pass through the membrane. Preferably, the poresize of the membrane is selective to the passage of water. The pore sizeof the membrane is preferably selected to prevent the flow of solute andother contaminants from the source solution to the draw solution.Typical pore sizes range from 1 to 100 Angstroms, preferably 5 to 50Angstroms, for example 10 to 40 Angstroms. Pore size may be inferred byusing any suitable technique.

The flow of solvent across a membrane is generally influenced by thermalconditions. Thus, the solutions on either side of the membrane may beheated or cooled, if desired. The solutions may be heated to highertemperatures of 40 to 90° C., for example, 60 to 80° C. Alternatively,the solutions may be cooled to −20 to 40° C., for example, 5 to 20° C.The solution on one side of the membrane may be heated, while the otherside cooled. The heating or cooling may be carried out on each solutionindependently. Chemical reactions may also be carried out on either sideof the membrane, if desired.

To improve the efficacy of the osmosis step, the draw and/or sourcesolution may be treated to reduce fouling and scaling of the membrane.Accordingly, anti-scaling and/or anti-fouling agents may be added to oneor both solutions. Although not required, pressure may be applied to thefirst solution side of the membrane to increase the rate of flux ofwater across the membrane. For example, pressures of 1×10⁵ Pa to 5×10⁵Pa [1 to 5 bar] may be applied, preferably pressures of 2×10⁵ Pa to4×10⁵ Pa [2 to 4 bar]. Additionally or alternatively, the pressure onthe second solution side of the membrane may be reduced. For example thepressure may be less than 1×10⁵ Pa [1 bar], preferably less than 0.5×10⁵Pa [0.5 bar].

The viscosities of the source solution and/or the draw solution may alsobe modified to improve the rate of flux across the membrane. Forexample, viscosity modifying agents may be employed.

The process of the present invention may further comprise apre-treatment step of removing contaminants, such as suspended particlesand biological matter, from the source solution. Additionally oralternatively, a threshold inhibitor to control scaling may be added tothe first solution. Pre-treatment steps to alter the pH of the firstsolution may also be employed. When seawater is used as a source, it ispreferable to use a deep sea intake, as deep seawater typically containsfewer contaminants.

After solvent (e.g. water) from the source solution has passed into thedraw solution, the draw solution may be at an elevated pressure (osmoticpressure when water is used as a solvent). This pressure may be used toaid the transfer of the draw solution to subsequent processing steps ofthe present invention. This pressure may be sufficient to transfer thedraw solution into the cooling tower, for example, without or with feweror less powerful pumps. In one embodiment, excess pressure is convertedinto mechanical work. Thus, the pressure (e.g. osmotic pressure)generated in the draw solution may be used to reduce the powerconsumption and/or increase the heat transfer efficiency of the overallprocess.

In one embodiment, the diluted draw solution may be contacted with oneside of a further membrane, while a further solution having a highersolute concentration than the diluted draw solution is contacted withthe other side of the membrane. As the diluted draw solution has ahigher solvent concentration than the further solution, solvent from thediluted draw solution flows across the membrane to dilute the furthersolution by direct osmosis. Like the initial draw solution, the furthersolution may consist essentially of a selected solute dissolved in aselected solvent. Thus, by repeating this direct osmosis step one ormore times, the composition of the solution introduced into the coolingtower may be better controlled.

The diluted draw solution from the direct osmosis step is preferablyused as the feed solution for the cooling tower. The cooling towerpreferably comprises supporting material or decking from which solvent(e.g. water) can evaporate. The supporting material is preferably porousand may advantageously have a large surface area. The supportingmaterial may be made from plastic, metal, ceramic and natural materials,such as wood.

In use, the feed solution is contacted with the supporting material. Agas, such as air, may then be passed through the wet supporting materialcausing the solvent of the feed solution to evaporate, forming aconcentrated feed solution. The temperature of the feed solution can bereduced as a result of the evaporative cooling. The cooled solution maybe used as a coolant in a heat exchanger, for example, to remove heatfrom a heat source.

The concentrated feed solution may be re-used as the draw solution toremove solvent from the source solution. Optionally, the concentratedfeed solution may be re-used as the draw solution after one or moreintermediate steps. For example, the concentrated draw solution may beused to remove heat from a heat source prior to being reused as a drawsolution. Alternatively, the draw solution may be used to remove heatfrom a heat source after it has been used to draw solvent from thesource solution in the direct osmosis step.

As evaporation occurs in the cooling tower, the feed solution becomesincreasingly concentrated and, after a period of use, contaminants maybuild-up in the feed solution. Such contaminants may include solutesand/or suspended components that flow across the membrane in the directosmosis step, for example, against the osmotic gradient. Examples ofsuch solutes include ions, such as sodium, calcium, magnesium,potassium, barium, strontium, chloride, sulphate, nitrate, bicarbonate,carbonate, bromide and fluoride ions. Particularly when present abovecertain concentrations, such solutes can cause scaling and/or fouling.Accordingly, to reduce the risk of scaling and/or fouling, a portion ofthe concentrated feed solution is removed from the cooling tower asblow-down.

In addition to unwanted contaminants and/or solutes, the blow-down maycontain desirable solutes (e.g. osmotic agents) and additives, such asscale inhibitors, corrosion inhibitors, biocides and/or dispersants. Forexample, in one embodiment, the blow-down contains desirable components,such as magnesium sulphate and, preferably, additive(s), such as scaleinhibitors, corrosion inhibitors, biocides and/or dispersants.

By selecting a membrane having appropriate characteristics, it ispossible to retain at least some of these desirable components on theretentate-side of the membrane, while allowing unwanted components, suchas sodium and chloride ions, to pass across the membrane together withthe solvent as filtrate. For example, in one embodiment, the blow-downcontains magnesium sulphate and sodium chloride. When the blow-down ispassed through the membrane, magnesium sulphate is largely preventedfrom passing through the pores of the membrane, while sodium andchloride ions are allowed to preferentially pass through the membrane.The filtrate solution, therefore, is a dilute solution of sodiumchloride, (with some magnesium sulphate) while the retentate is aconcentrated solution of magnesium sulphate (with some sodium chloride).

In a preferred embodiment, nanofiltration membranes are employed toretain some of the desirable constituents on the retentate side of themembrane, while allowing some of the undesirable constituents to passacross the membrane together with some of the solvent as filtrate.

Nanofiltration is particularly suitable for separating the large solutespecies of the blowdown from the remainder of the solution.

Suitable nanofiltration membranes include crosslinked polyamidemembranes, such as crosslinked aromatic polyamide membranes. Themembranes may be cast as a “skin layer” on top of a support formed, forexample, of a microporous polymer sheet. The resulting membrane has acomposite structure (e.g. a thin-film composite structure).

Typically, the separation properties of the membrane are controlled bythe pore size and electrical charge of the “skin layer”. The membranesmay be suitable for the separation of components that are 0.01 to 0.001microns in size and molecular weights of 100 mol−1 or above, forexample, 200 gmol−1 and above.

As well as filtering particles according to size, nanofiltrationmembranes can also filter particles according to their electrostaticproperties. For example, in certain embodiments, the surface charge ofthe nanofiltration membrane may be controlled to provide desiredfiltration properties.

For example, the inside of at least some of the pores of thenanofiltration membrane may be negatively charged, restricting orpreventing the passage of anionic species, particularly multivalentanions.

Examples of suitable nanofiltration membranes include Desal-5(Desalination Systems, Escondido, California), SR 90, NF 90, NF 70, NF50, NF 40 and NF 40 HF membranes (FilmTech Corp., Minneapolis, Minn), SU600 membrane (Toray, Japan) and NRT 7450 and NTR 7250 membranes (NittoElectric, Japan).

The nanofiltration membranes may be packed as membrane modules. Spiralwound membranes, and tubular membranes, for example, enclosed in a shellmay be employed.

Alternatively, the membranes may be provided as a plate or in a frame

The concentrated blow-down solution from the retentate-side of themembrane is contacted with one side of a second membrane, while asolution having a lower solute concentration than the concentratedblow-down solution with the opposite side of the second membrane, suchthat solvent flows across the second membrane to dilute the concentratedblow down solution by direct osmosis.

This direct osmosis step may be carried out using the same directosmosis unit used to draw solvent from the source solution into the drawsolution. Alternatively, this direct osmosis step may be carried out ina dedicated unit used to treat the blow-down stream. When a separatedirect osmosis unit is used, the direct osmosis membrane may have thecharacteristics mentioned above.

In any of the direct osmosis steps of the present invention, solventpreferably passes across the membrane in liquid form.

The diluted blow-down solution is then introduced into the coolingtower.

These and other aspects of the present invention will now be describedwith reference to the drawing which is a schematic diagram of a flowscheme for carrying out a process according to an embodiment of thepresent invention.

The drawing depicts a flow scheme for carrying out a process accordingto an embodiment of the present invention. The scheme depicts a coolingtower 10, a direct osmosis unit 12, a nanofiltration unit 14 and a heatexchanger 16.

The numbers on the drawing (FIG. 1) depict the following features:

30 Drift loss 32 Evaporation 34 Recirculated Osmotic Agent 36Concentrated Osmotic Agent 38 Reject from manipulated osmosis system 40Blowdown 42 Seawater or brackish water feed.

Seawater 18 is contacted with one side of a selective membrane in thedirect osmosis unit 12. A magnesium sulphate solution 20 containinganti-scaling agents is contacted with the opposite side of the selectivemembrane. The magnesium sulphate solution 20 has a higher soluteconcentration than the seawater and the difference in osmotic potentialbetween the two solutions causes water to flow across the membrane todilute the magnesium sulphate solution. Although sodium and chlorideions are largely prevented from passing across the membrane, some sodiumchloride passes across the membrane and this contaminates the magnesiumsulphate solution 20.

The diluted magnesium sulphate solution 20 is used as a coolant in aheat exchanger 16. The temperature of the magnesium sulphate solution 20increases as it removes heat from the system to be cooled.

The hot magnesium sulphate solution 20 is then introduced into thecooling tower 10. In the cooling tower 10, the magnesium sulphatesolution 20 is dripped or sprayed through decking, while air is blownthrough the decking, thereby evaporating water from the solution. Theevaporation lowers the temperature of the magnesium sulphate solution 20and the cooled magnesium sulphate solution 20 is recycled to the directosmosis unit 12. As more water is removed from the magnesium sulphatesolution, the solution becomes increasingly concentrated. Accordingly,the sodium chloride concentration in the magnesium sulphate solution 20increases and this can increase the risk of fouling in the system.

To reduce the risk of fouling, a portion of the concentrated magnesiumsulphate solution is removed from the cooling tower 10 as blow-down 22.The blow-down stream is passed through a membrane in the nanofiltrationunit 14. The pores of the nanofiltration membrane are sized to allowsodium and chloride ions to pass through the membrane as part of thefiltrate. Magnesium sulphate ions and the anti-scaling additives,however, are too large to pass through the pores of the membrane and areretained on the retentate side of the membrane. This concentratedretentate solution is introduced into the direct osmosis unit 12 andused to draw water from seawater by direct osmosis.

The filtrate from the nanofiltration unit 14 is discarded.

The invention claimed is
 1. A process for operating a cooling apparatus,said process comprising; preparing a draw solution by dissolving one ormore water-soluble salts in water; positioning a first membrane betweena source solution and the draw solution, wherein the draw solution isprepared having a higher solute concentration than the source solution,such that solvent and ions from the source solution flow across thefirst membrane to dilute the draw solution, the ions selected from thegroup consisting of sodium, calcium, magnesium, potassium, barium,strontium, chloride, sulphate, nitrate, bicarbonate, carbonate, bromideand fluoride ions; introducing the diluted draw solution into thecooling tower as a feed solution; evaporating, in the cooling tower,water from the feed solution to produce a concentrated solution having agreater concentration of ions than the diluted draw solution; removing(i) a portion of the concentrated solution as blow-down, and returning(ii) a portion of the concentrated solution to the cooling tower;passing at least a portion of the blow-down through a second membrane,such that some of the solute in the blow-down is retained as aconcentrated blow-down solution on the retentate-side of the secondmembrane, and at least some of the ions pass through the second membraneinto the filtrate side of the second membrane, and the filtrate with theions is discarded; contacting at least a portion of the concentratedblow-down solution with one side of the first membrane and contacting asolution having a lower solute concentration than the combinedconcentrated blow-down solution and the return from the cooling towerwith the opposite side of the first membrane, such that water flowsacross the first membrane to dilute the concentrated blow-down solutionby direct osmosis, and re-introducing the diluted blow-down solution tothe cooling tower.
 2. A process as claimed in claim 1, wherein thesource solution comprises seawater, brackish water, river water, lakewater and/or waste water.
 3. A process as claimed in claim 1, whereinthe salt is magnesium sulphate.
 4. A process as claimed in claim 1,wherein the second membrane is a nanofiltration membrane.
 5. A processas claimed in claim 1, wherein an additive selected from a scaleinhibitor, corrosion inhibitor, biocide and/or dispersant is included inthe draw solution.
 6. A process as claimed in claim 1, wherein the drawsolution is formed so as to be free from components that cause foulingof the cooling tower.
 7. A process as claimed in claim 1, wherein thefirst membrane is a direct osmosis membrane.
 8. A process as claimed inclaim 1 comprising introducing the feed solution into the top of thecooling tower; and dripping the feed solution through decking.
 9. Aprocess as claimed in claim 1 wherein the cooling tower comprisesdecking through which the feed solution drips, and the evaporating stepcomprises blowing air through decking; and causing some of the water inthe feed solution to evaporate.
 10. A process as claimed in claim 1comprising passing the diluted draw solution through a heat exchanger toheat the diluted draw solution; and introducing the heated diluted drawsolution into the cooling tower as the feed solution.